1. Field of the Invention
This invention relates generally to transmission and switching techniques in telephone communication systems and more particularly to a conference technique whereby a number of channels in a telephone switching system employing pulse code modulation for transmission purposes are combined so that a number of subscribers may participate in a common telephone conversation.
The present invention pertains to a conference circuit for use in a private automatic branch exchange similar to those units manufactured by GTE Automatic Electric Incorporated and designated GTD 120. Circuitry with minimum modification could also be employed in Class 5 central offices that employ digital switching. Such telephone systems employ a time switching network rather than the more prevalent earlier space divided switching network.
In time division switching networks a requirement exists to have sources of pulse code modulated voice samples associated with time slots. These time slots allow the conference to sequentially receive the code for each conferee. For the conference circuit to be effective it must be able to recognize who the conferees are and of course who is not associated with the conference. The circuitry must also be capable of distributing the conference speaker's code to each conferee. Information of this sort is of course available in the telephone switching systems referred to above. It should be understood that only telephone switching systems employing pulse code modulation can use the circuitry of the present invention and such circuitry interfaces with time division portions of such switching networks.
2. Description of the Prior Art
In the past, conferencing has generally been done using analog methods. Such methods usually require physical ports or terminations to a conference bridging circuit which then mixes analog signals at constant impedances. The result of such an arrangement is loss of the analog signal which increases as the number of ports increase. Such bridging arrangements usually employ amplifiers to increase the gain back to proper level. For example, a six port conference would have six terminations for lines or trunks which will be connected to the conferencing bridge. A bridge circuit designed for six terminations would then be employed with gain to adjust for the resultant losses. The loss, of course, being the result of six signals terminating on an impedance network to mix all the signals. In such an analog arrangement the signal would then be amplified and sent back to the six conferees.
Disadvantages of analog conferencing systems as outlined above include the cost of the terminations in the system, physical size and cost of the bridge circuitry and the problems resulting from varying levels as the number of conferees change. A variety of schemes for analog circuitry such as direct multiplexing, hybrid repeaters, transistorized analog circuitry, etc. have been utilized. Various solutions to the problem of cross talk, echo loss, impedance matching sidetone, etc. have been disclosed. Some of the techniques employed are shown in U.S. Pat. Nos. 3,116,369, 3,144,518, 3,170,042, and 3,301,962. None of the techniques disclosed in these patents however employ the digital approach and hence have any significance in the field of pulse code modulated telephony.
Another approach to the handling of pulse code modulated information in conference circuitry is taught by U.S. Pat. No. 3,699,264 which is assigned to the same Assignee as the present invention. In the noted patent digital signals are not converted to analog, rather binary words are compared from the participating channels, with the largest binary numbers selected as the speaker. The reference patent however utilizes a maximum binary code to select the speaker, while the technique of the present invention utilizes a minimum binary code as employed in the coding formats (D2 and D3) currently employed in pulse code modulated telephony. Other differences which result in substantial economy of equipment are also included in the present invention. Such improvements will be obvious from the following specification.